Image pickup device and camera for converting charges into voltage

ABSTRACT

An object is to provide a solid state image pickup device and a camera which do not worsen a sensor performance in terms of an optical property, a saturated charge amount and the like. A solid state image sensor including a pixel region having a plurality of pixels includes at least a photodiode and an amplifying portion amplifying photocharges outputted from the photodiode in the pixel region, and further includes a well electrode for taking well potential of a well region in which the amplifying portion is arranged. Between the well electrode and the photodiode, no element isolation regions by an insulation film are arranged. Moreover, on the surface of a first semiconductor region in which the photodiode stores the charges, a second semiconductor layer of a conductivity type reverse to that of the first semiconductor region is arranged.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image pickup device and a camera,and more particularly to an image pickup device and a camera in whichcharges are converted into a voltage in a pixel region to be read as avoltage signal like an active pixel sensor (APS).

2. Description of Related Art

In recent years, a demand for a digital single lens reflex camera hasbeen developing, and a sensor used in the digital single lens reflexcamera is sized to be large one from APS-C size to a 35 mm film size.

Moreover, the used sensor itself widely varies from a CCD to an APS andthe like.

Japanese Patent Application Laid-Open No. 2001-230400 (correspondingU.S. application was published as U.S. Publication 2001012133A)discloses an amplifying type image pickup device having a plurality oftwo-dimensionally arranged pixels, each including a photoelectricconversion element and an amplifying transistor, wherein: a firstconductivity type semiconductor region constituting each photoelectricconversion element is formed in a common well composed of a secondconductivity type semiconductor formed in a first conductivity typesemiconductor substrate; a first conductivity type semiconductor regionconstituting a source and a drain of each of the amplifying transistorsis formed in the common well; and a plurality of electric contacts forsupplying a reference voltage to the common well is provided in theinside of the pixel array area in the common well.

In the APS sensor which has the large image pickup surface mentionedabove, it is necessary to perform a voltage conversion of charges basedon certain reference potential. When the reference potential isdistributed on the image pickup surface, also the optical signal havingreceived the voltage conversion has shading according to thedistribution. Thus, there is a problem in which image performance isseriously damaged.

For coping with the problem, the prior art disclosed in the publicationmentioned above provides an electrode for fixing the potential of thewell in which a source follower is arranged to the reference potentialwhen the source is used as an amplifying portion in a pixel, forexample.

FIG. 10 is a sectional view showing the cross-sectional structure ofprior art. In FIG. 10, an electrode region for taking well potential isdenoted by a reference numeral 2.

A reference numeral 1 denotes n-type semiconductor region forming aphotoelectric conversion region. A reference numeral 2 denotes p-typesemiconductor region. A reference numeral 3 denotes a well contactwiring. A reference numeral 4 denotes P-well. A reference numeral 12denotes an element isolation region. Reference numerals 13–17 denotesource and drain regions of MOS transistor. A reference numeral 101denotes a photoelectric conversion unit. A reference numeral 102 denotesa transfer MOS transistor. A reference numeral 103 denotes a reset MOStransistor. A reference numeral 104 denotes a selection MOS transistor.And, a reference numeral 105 denotes an amplifying MOS transistor.

FIG. 11 is a plan view showing the planar structure of the prior art.The well region 2 of FIG. 10 corresponds to a well electrode 1101 inFIG. 11.

A reference numeral 1104 denotes a photodiode. A reference numeral 1102denotes a poly wiring (poly gate). A reference numeral 1103 denotes aMOS transistor unit (N⁺). L_(min) denotes element isolation width. And,S_(min) denotes an area of the well electrode 1101.

Generally, between a well region and a light receiving unit, aninsulating isolation region represented by the localized oxidation ofsilicon (LOCOS), the shallow trench isolation (STI) and the like isarranged. In FIG. 10, LOCOS 12 is arranged as such an element isolationregion.

In arranging the element isolation region, the following restrictionsare especially given from a viewpoint of an exposure process onproduction. Those are (1) the securement of the minimum separation widthLmin and (2) the securement of the minimum electrode area Smin.

In performing the miniaturization of a pixel, by the restrictionsmentioned above, the light receiving area of a photodiode used as thelight receiving unit becomes small, and the sensor performance has beenworsened in terms of the optical property thereof, the saturated chargeamount thereof, and the like.

Accordingly, it is an object of the present invention to provide a solidstate image pickup device and a camera which do not worsen the sensorperformance in terms of the optical property thereof, the saturatedcharge amount thereof, and the like.

SUMMARY OF THE INVENTION

As means for solving the problem mentioned above, the present inventionis an image pickup device having a plurality of photoelectric conversionelements, and a plurality of amplifying elements amplifying charges fromthe photoelectric conversion elements, both elements arranged in a firstconductivity type well. The device includes a first semiconductor regionof the first conductivity type for regulating potential of the well; anda protection layer arranged to cover a light receiving surface of thephotoelectric conversion element excluding the first semiconductorregion.

Moreover, the present invention is a solid state image pickup deviceincluding a pixel region having a plurality of pixels on a semiconductorsubstrate, the solid state image pickup device provided with at least alight receiving unit and an amplifying portion amplifying photochargesfrom the light receiving unit in the pixel region, and further providedwith a first semiconductor region for regulating potential of a wellregion, in which the amplifying portion is arranged, and a siliconnitride film arranged between the semiconductor substrate and a wiringlayer so as to cover at least a part of the light receiving unit,wherein the silicon nitride film is not arranged on the firstsemiconductor region.

Moreover, the present invention is characterized by an optical system, adiaphragm limiting an amount of light pas sing the optical system, andthe above-mentioned solid state image pickup device receiving the lightpassing the diaphragm.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a first embodiment of the present invention;

FIG. 2 is a sectional view of the first embodiment of the presentinvention;

FIG. 3 is a sectional view of a second embodiment of the presentinvention;

FIG. 4 is a sectional view of a third embodiment of the presentinvention;

FIG. 5 is a plan view of the third embodiment of the present invention;

FIG. 6 is a sectional view of a fourth embodiment of the presentinvention;

FIG. 7 is a sectional view of a fifth embodiment of the presentinvention;

FIG. 8 is a plan view of the fifth embodiment of the present invention;

FIG. 9 is a view showing an example of the circuit block in case ofapplying a solid state image pickup device according to the presentinvention to a camera;

FIG. 10 is a sectional view showing the cross-sectional structure ofprior art; and

FIG. 11 is a plan view showing the planar structure of the prior art.

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the best embodiments for implementing the presentinvention are described with reference to the attached drawings.

First Embodiment

FIG. 1 is a plan view of a first embodiment of the present invention,and FIG. 2 is a sectional view of the first embodiment of the presentinvention.

The present embodiment is an image pickup device including photodiodeseach composed of an N type semiconductor region 202 formed in a P typesemiconductor region (well).

In FIGS. 1 and 2, a reference numeral 101 denotes a well regionconnecting semiconductor region (P type semiconductor region). Areference numeral 102 denotes a photodiode to become a photoelectricconversion element. A reference numeral 103 denotes a transfer switchtransferring charges from the photodiode 102. A reference numeral 104denotes a charge conversion unit for converting a signal charge into avoltage. A reference numeral 105 denotes a metal oxide semiconductor(MOS) transistor unit provided according to uses such as the resettingof the charge conversion unit and the amplification of a signal charge.The reference numeral 202 denotes the N type semiconductor regionconstituting the photodiode. In fact, an electrode is connected to thewell electrode 101 through a contact hole. The well electrode 101 isformed for regulating the potential of a well in which an amplifyingelement for amplifying the signal charge is formed.

As shown in FIG. 2, no element isolation regions such as the LOCOS arearranged between the well electrode 101 and the photodiode 102. That is,the well electrode 101 is arranged in the same active region as thephotodiode 102.

On the other hand, an element isolation region 106 by the LOCOS isarranged between the MOS transistor units 105.

In order to avoid a dark current originated in a defect of a LOCOS end,the N type semiconductor region 202 is arranged to be away from theLOCOS end.

According to the present embodiment, because no element isolationregions by the LOCOS is arranged between the well electrode 101 and thelight receiving unit of the photodiode 102, it becomes unnecessary toform any offset region for suppressing the dark current. Instead of theelement isolation region, an offset region is formed between the N typesemiconductor region 202 and the well electrode 101.

The reason is that, when the N type semiconductor region 202 and thewell electrode 101 are touched with each other directly, the touchbecomes a cause of the dark current.

As a result, the space of the amount of the distance of the elementisolation region 106 can be saved. The amount of the distance of theelement isolation region 106 is 0.7 μm in the present embodiment.

Moreover, in the prior art, 2 μm² of the area has been required as thewell region.

However, according to the present embodiment, because the active regionfor the well region is connected to the active region of the photodiode,the active region for the well region may satisfy to be 2 μm² or more intotal, and it becomes possible that the area of the impurity region forthe well region is even 0.64 μm².

As a result, the space of 1 μm² or more can be decreased owing to theeffect of the configuration.

Second Embodiment

FIG. 3 is a sectional view of a second embodiment of the presentinvention. The same reference characters as those in the firstembodiment are given to the portions having the same functions as thoseof the first embodiment, and their descriptions are omitted.

The present embodiment differs from the first embodiment in that thepresent embodiment provides an image pickup device in which the buriedphotodiode 102 is formed in a P type semiconductor region.

The buried photodiode structure is constructed by arranging a P typesemiconductor region 507 on the surface of the photodiode. As isapparent from the drawing, the well region is arranged up to a deeperposition in the semiconductor substrate in comparison with the P typesemiconductor region on the surface on the basis of a principal surfaceof the semiconductor substrate on which the photodiode is arranged, as areference.

Also in the present embodiment, because the element isolation region 106by the LOCOS is not arranged between the well electrode 101 and thephotodiode 102, it becomes unnecessary to form the offset region forsuppressing the dark current, and an offset region is formed between theN type semiconductor region 202 and the well electrode 101 instead.

As an effect of the space saving according to the present embodiment,1.7 μm can be obtained.

Third Embodiment

FIG. 4 is a sectional view of a third embodiment of the presentinvention.

The present embodiment differs from the second embodiment in that thepresent embodiment forms the P type semiconductor region to extend overthe well region. That is, a surface P region 606 is arranged as shown inFIG. 5. As is apparent from the drawings, the well region is arranged upto a deeper position in the semiconductor substrate in comparison withthe P type semiconductor region on the surface on the basis of aprincipal surface of the semiconductor substrate on which the photodiodeis arranged, as a reference.

By adopting such a configuration, it becomes possible to suppress theunevenness of dark currents.

The dark current of the photodiode 102 contains a generation currentcomponent generated from a defect existing in a depletion layer, and adiffusion current component generated from the density differencebetween electrons and holes in a PN junction surface. As one of thesources of the diffusion current component, there is the well electrode101.

That is, the position of the well electrode 101 and the impurity densityfrom the well electrode 101 to the N type semiconductor region 202 isone of the primary factors determining the diffusion current component.

As the second embodiment, when a surface P region 307 is stopped at an Xposition in the drawing, i.e. when the surface P region 307 is formed asshown in FIG. 3, the position of the surface P region may overlap or maynot overlap with the well electrode 101 in a surface according to theregistration accuracy (shifts in the X direction, the Y direction andthe Θ direction) and the dispersion of dimensions. That is sometimesseen as unevenness of dark currents.

That is, the density of the area between the well electrode 101 and theN type semiconductor region 202 may differ to every pixel, and it may beseen as unevenness. The unevenness becomes several percents of order ofmagnitude. Although there is no problem when a dark current valuefluctuates to this extent on the whole, the unevenness may becomeconspicuous when a dark current difference arises at some positions inthe same image pickup device.

In the present embodiment, because the surface P type semiconductorregion 307 is extended to the well electrode 101, the dispersion of thedensity in the area between the well electrode 101 and the N typesemiconductor region 202 in every pixel becomes nonexistent.

As a result, about 1.7 μm of pixel reduction can be obtained, and thedark current unevenness can be also suppressed.

Fourth Embodiment

FIG. 6 is a sectional view of a fourth embodiment of the presentinvention.

The present embodiment differs from the third embodiment in that no P⁺regions formed by a P⁺ ion (for example, boron) implantation process forforming the sauce and the drain regions of a general MOS as the wellelectrode 101 are not used. Instead, after forming a contact hole forconnecting a metal region with a semiconductor region, the wellelectrode 101 is formed by performing P⁺ ion implantation using thecontract hole as a mask. As is apparent from the drawing, the wellregion is arranged to a deeper position of the semiconductor substratecompared with the surface P type semiconductor region on the basis of aprincipal surface of the semiconductor substrate on which the photodiodeis arranged, as a reference.

Thereby, the space of the electric contact and the margin of an activeregion of about 0.2 μm can be saved. As a result, 1.9 μm of the pixelreduction can be achieved.

Fifth Embodiment

FIG. 7 is a sectional view of a fifth embodiment of the presentinvention.

The feature of the present embodiment is to form a photodiode protectionfilm 407 for protecting a photodiode from the damage of an etch backprocess of a lightly doped drain (LDD). As is apparent from the drawing,the well region is arranged to a deeper position of the semiconductorsubstrate compared with the surface P type semiconductor region on thebasis of a principal surface of the semiconductor substrate on which thephotodiode is arranged, as a reference.

In the present embodiment, the etch back process of the LDD is performedin a state in which a resist film is deposited at the position of aphotodiode protection film 407 at the time of the LDD etch back.

FIG. 8 is a plan view of the present embodiment. A structure in which anoxide film for a LDD remains on a photodiode is obtained.

The photodiode protection layer is arranged in the region surrounded byan alternate long and short dash line 707 in the drawing.

In the present embodiment, the well electrode 101 is formed in an areabordered by the protection layer.

That is, a resist aperture portion for performing P⁺ ion implantationfor forming the well electrode 101 is made to extend also to a part ofthe photodiode protection layer.

Hereby, all of the active regions receiving etch back damages are madeto be well regions.

According to the configuration of the present embodiment, space savingcan be achieved, and the dark current unevenness described with regardto the third embodiment is also improved. The reason is as follows.

An active region having received damages owing to the registrationaccuracy (shifts in the X direction, the Y direction and the Θdirection) of the P⁺ semiconductor region for a well region and thedispersion of dimensions in the well region surface may exist or may notexist in an area between the well electrode 101 and the N typesemiconductor region 102. Then, those damaged active regions can be seenas the unevenness of the dark current.

That is, because the damaged state of the semiconductor region betweenthe well electrode 101 and the N type semiconductor region 202 differfrom each other in every pixel, the damaged regions may be seen as theunevenness.

Because the resist aperture portion for the P⁺ ion implantation forforming the well electrode 101 is made to extend also on a part of thephotodiode protection layer from the reason mentioned above, theunevenness of the dark current can be more suppressed.

Sixth Embodiment

The photodiode protection layer is formed as follows in the fifthembodiment.

An 8 nm of a silicon oxide film, a 50 nm of silicon nitride film, and a500 nm of a silicon oxide film are formed from the surface of thesemiconductor in the order.

As a result, in addition to the effect of the fifth embodiment, thelaminated structure of the silicon oxide film and the silicon nitridefilm performs an anti-reflection function, and the improvement insensitivity can been also improved by about 20%.

Seventh Embodiment

FIG. 9 shows an example of a circuit block in the case of applying animage pickup device according to the present invention to a camera.

A shutter 1001 is provided before a photographing lens 1002, and theshutter 1001 controls exposure. A light amount is controlled by adiaphragm 1003 as the need arises, and the light is made to performimage formation on an image pickup device 1004.

A signal outputted from the image pickup device 1004 is processed by asignal processing circuit 1005, and is converted into a digital signalfrom an analog signal by an A/D converter 1006.

The operation processing of the digital signal outputted from the A/Dconverter 1004 is further performed by a signal processing unit 1007.

The processed digital signal is stored in a memory 1010, or istransmitted to an external apparatus through an external I/F unit 1013.

The image pickup device 1004, the image signal processing circuit 1005,the A/D converter 1006, and the signal processing unit 1007 arecontrolled by a timing generator 1008, and also the whole system iscontrolled by a unit controlling whole and arithmetic operation unit1009.

In order to record an image on a recording medium 1012, an outputdigital signal is recorded through an I/F unit controlling recordingmedium 1011 controlled by the unit controlling whole and arithmeticoperation unit 1012.

This application claims priority from Japanese Patent Application No.2004-254362 filed on Sep. 1, 2004, which is hereby incorporated byreference herein.

1. An image pickup device having a plurality of photoelectric conversionelements, and a plurality of amplifying elements amplifying signal basedon charges from the photoelectric conversion elements, both elementsarranged in a first conductivity type well, the device comprising: afirst semiconductor region of the first conductivity type for regulatingpotential of said well; and a protection layer arranged to cover a lightreceiving surface of the photoelectric conversion elements except for aregion over the first semiconductor region, wherein said firstsemiconductor region is arranged in an active region of saidphotoelectric conversion element.
 2. An image pickup device according toclaim 1, wherein the protection layer contains silicon nitride.
 3. Animage pickup device according to claim 1, further comprising anelectrode electrically connected to said first semiconductor region. 4.An image pickup device including a pixel region having a plurality ofpixels on a semiconductor substrate, a photoelectric conversion element,an amplifying element amplifying signal based on charges from thephotoelectric conversion element, a first conductivity type firstsemiconductor region for regulating potential of a first conductivitytype well, in which the amplifying element is arranged, and a wiringlayer arranged on the semiconductor substrate, the photoelectricconversion element, the amplifying element, the first semiconductorregion and the wiring layer included in the pixel region, the devicecomprising: a silicon nitride film arranged to cover the photoelectricconversion element between the semiconductor substrate and the wiringlayer, and not to cover the first semiconductor region.